On page 51, when discussing abstract classes, it says that writing default implementations for all methods in abstract classes is not possible using C++'s pure virtual syntax.

That's not true.

Try the following C++:

--------8<--------
#include <iostream>

struct base {
    virtual void f() = 0;
};

void base::f()
{
    std::cout << "base::f() called\n";
}

struct derived : public base {
    void f();
};

void derived::f()
{
    std::cout << "derived::f() called\n";
    base::f();
}

int main()
{
    derived d;
    d.f();
}
--------8<--------

You can define default implementations for PVFs.